Production of silicon



May 23, 1961 D. G. KELEMEN PRODUCTION OF SILICON Filed June 2, 1958 INVENTOR DEN/S a. KEL EMEN ATTokNEY United States Patent PRODUCTION OF SILICON assignor to E. I.

Denis G. Kelemen, Wilmington, DeL,

Del.,

du Pont de Nemours and Company, Wilmington, a corporation of Delaware Filed June 2, 1958, Ser. No. 739,430 7 Claims. (Cl. 2$301) single crystal.

High purity silicon is an excellent semiconductor, and for this reason, it is valuable in the manufacture of electronic devices, such as transistors. One of the most successful processes for producing high purity silicon is by the vapor phase reduction of silicon tetrachloride with zinc. The silicon obtained by this reduction is usually in the form of needle-like or acicular particles which are melted and solidified in a silica crucible to form larger solid objects which are more practical for use by the semiconductor manufacturer. This melting and solidification would appear to be a relatively simple process, but in fact, it is attended by many difficulties. The crucible usually shatters and breaks during the cooling. In addition to this, oxide impurities in the silica of the crucible are reduced by the molten silicon and transferred to the melt. Another complication is that silica from the crucible adheres to the surface of the solidified silicon. This adhering silica must be dissolved away by hydrofluoric acid, and even though highly pure acid is used, it is still possible to encounter undesirable contamination. For most uses, such minute contamination would be negligible but for semiconductor purposes, every atom of an undesirable impurity degrades the silicon.

It is, therefore, an object of this invention to provide a. process for melting and solidifying silicon without encountering undesirable contamination of the silicon. It is another object to adapt this process to melting and solidifying the silicon into a single crystal. It is a still further object of this invention to provide a continuous process for melting and solidifying silicon.

These and other objects are accomplished by the process described below. First, silicon is made sufliciently electrically conductive to be quickly heated by the energy of an electromagnetic field. This may be accomplished in several ways. For example, the silicon may be preheated to temperatures above about 700 C., or it may be subjected to radiation from high intensity light, beta rays or gamma rays. The electrically conductive silicon is then placed in the electromagnetic field of an induction coil and direct inductive coupling takes place to melt the silicon. Situated within the induction coil and its field is a walled enclosure which can be kept below the melting point of silicon by having a fiuid coolant, such as water, contacting its exterior surface. Both the coolant and the enclosure are composed of nonconducting (dielectric) materials so that they are substantially unaffected by the electrical field. The silicon may be introduced into the electromagnetic field (i.e., the melting zone) within the walled enclosure from either the bottom or the top end of said enclosure. When the silicon is to be fed upwardly, the electrically conductive end of a silicon rod slightly smaller but similar in shape to the lower end of the enclosure is fed into the electromagnetic field and melted by the fields energy. At the same time this rod is being fed and melted, the unmelted por- Patented May 23, 1961 'ice 7 if the height of the pool and the space are kept small enough to allow surface tension to keep the silicon in the melting zone. In instances where surface tension alone is unable to hold the molten silicon in the melting zone, a noncontaminating gas can be used to exert a supportingpressure on the bottom of the pool. Once melting is initiated, it is'no longer necessary to actively cause the silicon to become electrically conductive since the process is inherent as a result of the heat transferred to the rod from the melting zone. When suflicient silicon has been fed and melted to form a pool of liquid silicon, a seeding crystal (or crystals) is touched to the upper surface of the molten silicon and then slowly withdrawn to a cooling zone. This seeding process initiates crystallization on the seed, and it is continuous so long as silicon is fed into the electrical field and the crystallizing silicon is withdrawn. In this invention, molten silicon can be solidified out of contact with a mold or other containing means, and this makes it possible to have all of the solidified silicon as a single crystal. To obtain a single crystal, a single crystal seed is used; and as it is withdrawn from the molten silicon, there is a continuous growth in the habit of the seed. A polycrystalline rod can be grown by using a polycrystalline seed. The complete operation is carried out in an inert atmosphere since at the elevated temperatures of the operation air would react with the silicon.

If the process is to be operated by feeding silicon to the electromagnetic field from above, it is necessary to support the bottom of the molten pool. This may be done initially by maintaining a support of silicon of the proper size below the melting zone. When a pool of molten silicon is obtained in the manner already described, the rod is withdrawn downwardly into a cooling zone to allow molten silicon to elongate the support by continually crystallizing thereon.

The above-described process provides a continuous operation for melting silicon, and it is' adapted to produce silicon rods of large diameter. These rods may be of the same size as the rod which is melted, or if desired a larger or smaller rod can be produced. Previous to this invention such rods were produced in a batch process requiring a silica mold which is usually weakened by the high temperature of the melt and then destroyed during the expansion of the silicon upon freezing. This invention makes it possible to solidify silicon out of contact with a mold or other containing means, and without the necessity of using expendable equipment. Furthermore, the use of a nonconductingfiuid-cooled wall in the melting zone not only lengthens the life of the .wall, but it minimizes the danger of having impurities from the wall react with the silicon thus causing contamination. I

Referring now to the drawing, there is shown the process of this invention in operation. A silicon rod 1 (composed of compressed particles of elemental silicon from the vapor phase reduction of silicon tetrachloride by zinc) is rotated and continuously fed upwardly through entrance zone 2 and into the high frequency electromagnetic field of induction coil 3 by a feeding device (not shown). Entrance zone 2 is enclosed by a tube 4, preferably constructed of clear silica, which communi cates with melting zone 5 at its upper end. Tube 4 is provided with inlet 6 and outlet 7 so that an inert gas, such as argon, may be flowed through it. A similar but longer tube 10 is provided above silica wall 8, and it en-' closes the cooling zone 9. This tube is preferably of silica, and .it is provided with inlet 11 and outlet 12 so .that an inert gas, such as argon, can be flowed through the tube when the apparatus is in operation. Silica wall 8 is surrounded by a nonconducting jacket 13 preferably made of clear silica which extends equi-distant along tubes 4 and 10, said jacket having the function of contacting walls 4, 10 and 13 with a fluid nonconducting coolant, such as water.

Under the influence of the high frequency field of the induciton coil 3, the rod being fed upwardly into the field is continuously melted. The molten pool of silicon thus obtained is detained by the surrounding water-cooled wall 8, and is supported from below by the pressed silicon rod 1 and by surface tension. In instances where surface tension alone will not prevent molten silicon from flowing into tube 4, a noncontaminating gas is maintained in this tube at a pressure which is sutficiently higher than the gas pressure in cooling zone 9 that there can be no downward flow of silicon from the molten pool. After the pool is fully established, its volume can be maintained substantially constant by withdrawing the silicon rod 14 at a sufficient rate to remove from the pool a quantity of silicon equal to that being introduced by the melting of moving rod 1.

The end of rod 1 continuously becomes sufliciently electrically conductive to inductively couple and melt readily in the melting zone 5 by heat transfer from the melt 5. The melt 5 is solidified and withdrawn continuously as a single crystal rod in the habit of the initial seed crystal. The solidified single crystal in rod form is cooled in the argon atmosphere until it is unreactive with the normal atmosphere in zone 9, and continuously removed from the apparatus as a rod.

In starting the melting process of the invention, it is preferred to initially bring the temperature of the silicon rod to withinthe range of 700-1100 C. so that its electrical conductivity is relatively high and it can directly couple in the field of the high frequency induction heating coil 3. This may be done by placing a narrow ring of tantalum wire around the seed crystal, before it is lowered into the apparatus. The ring is placed about 3 inches above the end of the seed crystal, then positioned in the melting zone 5. Initially, the tantalum ring is heated rapidly by induction which in turn transfers heat to the adjacent silicon causing it to accept the energy of the induction coil 3. This changeover is observable from .outside the clear silica tubes because of the extension of the glowing zone to the seed crystal rod inside the coil. At'this time the feed rod is raised up to a position in contact with the single crystal and both rods raised to bring the junction within the melting zone 5 and the power on the coil is then increased to melt the siliconwithin the zone 5.

The apparatus shown in the drawing is for purposes of illustration only, and it is subject to a number of modifications. For example, one modification which. is particularly desirable for preparing large diameter crystals consists of having water in contact with only that portion ofthe wall 8 which surrounds the melting zone, while silicon rods 1 and 14 are kept just below the melting point by separately controlled'induction coils on either side of the melting zone. The purpose of this heating is to control the dissipation of heat at solid-liquid interfaces 15 and 16. If heat is dissipated too rapidly from the center of one or the other of these interfaces a. bridge of solid silicon will form and connect the two rods through the molten zone. This is prevented in the apparatus shown in the drawing by using an induction coil which provides a deep melting zone.

The following examples illustrate the invention in detail. These examples are intended to be merely illustrative of the invention and not in limitation thereof.

Example I A single crystal rod of pure silicon about 6" long and approximately 2" in diameter was used as the seed for crystallizing silicon from high purity silicon. A thin Vs" tantalum band was secured around the center of the seed crystal and lowered through zone 9 into the melting zone 5 of the apparatus shown in the figure. 'IIhis melting zone 5 (2%" in diameter and 2." high) was surrounded by a 5-turn induction coil 3" in diameter ad 2" high. Power for the coil was supplied by a 450 kilocycle, -kw. vacuum tube oscillator. A 2" diameter rod of high purity silicon (made by compressing small particles of silicon from the vapor phase reduction of SiCl; with zinc) was inserted upwardly through entrance zone 2 to a position just in contact with the lower end of the seed crystal. Argon gas was then fed to zones 2 and 9 and the power supplied to the induction coil. The silicon adjacent to the tantalum band was heated to a temperature of about 800 C. by heat transfer from the tantalum band which was coupling with the inducted energy. At this point the two rods were moved upwardly through the melting zone, and heating of the silicon was continued with water circulating through jacket 13 at the rate of about 10 liters per minute. When the interface bet-ween the two rods was centered in the melting zone 5, thepower was increased until melting was observed. Argon pressure below the melt was increased to about 7 mm. (Hg) above that of the argon over the melt. The wall 8 of the melting zone 5 was maintained below 1100 C. by the fiow of water on its outside surface. The rod of compressed silicon was fed upwardly at the rate of about 4" per hour and was rotated at about 15 r.p.m. The single crystal rod 14 formed above the molten zone 5 as the initialseed crystal rod was withdrawn at a rate of about 4" per hour through the cooling zone 9. A solid, single crystal rod of high purity silicon sutliciently cooled to be unreactive with normal atmosphere was recovered. This silicon was suitable for use in the manufacture of diodes, transistors, rectifiers, and other electronic devices.

- Example II Silicon crystals obtained by the vapor phase reduction of silicon tetrachloride with zinc vapor was doped with 20 p.p.m. of boron, and compressed to form a rod. The compressed crystalline rod was melted and solidified in the manner described in Example I. The product obtained was a single crystal rod of silicon with a uniform boron content throughout and it was suitable for P type electrical semiconductor uses.

Example III A cylindrical single crystal of hyperpure' silicon, having a diameter of two inches and a length of three inches is placed in the lower part of the continuous melting and solidifying apparatus shown in the drawing. The top of the crystal is placed at a point just above the bottom of the melting zone. A polycrystalline rod of pure semiconductor grade silicon and one-half inch in diameter is inserted from the top to a point almost touching the twoinch crystal. A tantalum ring is placed around the end of the polycrystalline rod, and after purging the apparatus with argon, the ring is heated by induction to indirectly heat the end of the polycrystalline rod to a red heat; The tantalum ring is then removed, and the end of the hot polycrystalline rod is then melted by direct induction heating. Thev flow of cooling water in the jacket of the silica apparatus is started as soon as melting begins, and as the polycrystalline silicon melts, the rod is fed downward into the melt. The argon pressure below the melt is increased about 10 mm. (Hg) above the pressure of the argon over the melt, in order to prevent molten silicon from flowing out of the bottom of the melting zone. The molten zone is increased in size until it is two inches high, and then the crystalline seed is withdrawn at the rate of 4 inches per hour. The molten zone is maintained at a constant volume by feedingtheone-halfinchrod atarateof 16inchesperhour.

The silicon rod produced is pure single crystal .ilicon of semiconductor quality.

Example IV Using the apparatus shown in the drawing, a polycrystalline rod 2 inches in diameter is fed upwardly into the melting zone at a rate of 4 inches per hour, while cooling the melting zone walls by flowing water through the surrounding jacket at the rate of liters per minute. Argon is fed to the upper and lower sections, and the pressure in the lower section is'kept at about 9 mm. (Hg) above that in the upper. The molten zone is maintained at a depth of two inches, and a single crystal rod of silicon one quarter of an inch in diameter is touched to the surface of the melt and withdrawn at a rate of 64 inches per hour. Since the silicon used to start the withdrawal from the melt is in single crystal form, the thin rod produced is a continuation of this single crystal.

As can be seen from-Examples III and IV the invention is suited for producing silicon rods of either larger or smaller cross-section than the feed rod. Example III illustrates the invention when it is used to produce a larger (2 diam.) rod from a smaller /z" diam.) feed rod. On the other hand, Example IV produces a smaller 04 diam.) rod from a much larger (2" diam.) feed rod. Controlof cross-sectional size of the rod being withdrawn is accomplished by controlling the lineal rate of withdrawal. The faster the withdrawal, the smaller the cross-sectional size. When the rod being withdrawn is of a diflferent cross-sectional size than the feed rod, the volume of the molten pool is kept constant by adjusting the rate at which the feed rod is introduced into the electromagnetic field. This rate should be suflicient to supply silicon to the pool at the same volume rate as it is being withdrawn.

The dielectric refractory materials suitable for use in the wall of the melting zone include silica, alumina, mullite, zirconia, thoria and the like. The preferred material for handling semiconductor silicon is silica of the purest obtainable grade. The materials for construction of the walls of the introductory and cooling zones are of a less critical nature because these walls do not contact the silicon. Silica is still the preferred material here, but the other refractory materials enumerated above may be used.

The shape of the nonconducting wall of the molten zone can be varied as desired. While cylindrical walls in vertical alignment, as shown in the drawing, are preferred for most applications, the wall may be shaped as a truncated cone. In this way the top of the molten pool can be of different diameter than the bottom, and when the smaller diameter is at the bottom, a rod of similarly small dimension can be fed thereto, and a larger diameter rod withdrawn at the top. When such a truncated cone-shaped melting zone is used, the shape of the induction coils can be adjusted accordingly to obtain more effective heating.

The process of this invention should be conducted in a noncontaminating environment if a product of maximum purity is desired. This noncontaminating environment can be an inert atmosphere, such as a helium or argon atmosphere; a noncontaminating gas such as hydrogen; or mixtures of noncontaminating gases. Furthermore, after the molten pool has been established, the noncontaminating atmosphere used above the pool may be difierent from the atmosphere used below.

The maintenance of a greater gas pressure below the melt than above it can be easily accomplished by one skilled in the art. The specific pressures used are not critical as long as the greater pressure below the melt is sufiicient to prevent a downward flow of silicon. In approximating this pressure difierential, the following equation can be used:

6 where AP is the pressure diflerential in millimeters of mercury, h is the height of the molten silicon in millimeters, d is the density of silicon at the melting point, and d is the density of mercury at the melting point of silicon. When using silica equipment which is sufliciently transparent to observe'the molten silicon, pressure adjustments can be made in accordance with visual observations of the behavior of the molten pool.

The dielectric coolant fluids utilizable for flowing overthe exterior wall of the melting zone include water, air, liquid nitrogen, and fluidized silica powder in air. The preferred coolant is water because of its high heat capacity and low cost. Another good coolant is silica powder fluidized with air. The rate of flow of coolant is regulated to maintain the melt-contacting wall of the melting zone at a temperature below the melting point of silicon and preferably below about 1100 C. The use of a double wall to form an annular space around the actual melt-containing tube and the circulation of the coolant through this annular space is the most effective cooling procedure. However, it is not necessary to confine the coolant in this manner. If desired, a film of water or other coolant can be contacted with the exterior surface of a single wall which encloses the melting zone;

In the drawing, the nonconducting cooling jacket 13 is a single-jacket which surrounds the melting zone and also the entrance zone 2 and cooling zone 9. This arrangement is subject to modifications which permit a difierent rate of cooling in each of the three zones. For example, the annular space within the jacket may be divided into' three or more separate zones. Difierent cooling rates can be utilized in the diiferent zones, by circulating the same coolant at different rates. Furthermore, coolants having different heat capacities or melting points may be used in the diflerent sections of the annular space.

The high frequency generating equipment for the in duction coil will vary with the conditions of operation. For example, with up to a diameter melting zone, a vacuum tube oscillator generating alternating current at a frequency of 3 megacycles is used; with a melting zone with a diameter ranging from about A" to 3", a vacuum tube oscillator or a spark gap generator delivering current at a frequency of 450 kilocycles is used; for melting zones larger than 3" in diameter, a spark gap or motor generator delivering current at a frequency of 5-50 kilocycles is used.

The type of feed used for the melting and crystallizing process of the invention can be varied. For example, solid particles of the element material may be compressed or previously molded to form a rod of slightly smaller diameter than the tubular melting zone. If 'desired, granular or particulate feed may be used. vFor continuous melting with granular or particulate silicon, the melting zone is fed from above and the crystallized rod removed continuously from below the melt. When a rod type feed is used it may be fed from above or below.

Elemental silicon of very high purity is usually used as the feed for this type of melting and solidification process, and silicon obtained from the vapor phase reduction of silicon tetrachloride is preferred since it can be obtained in high purity. It will be seen from Example II that doped silicon may also be used as a feed material. Doped silicon is a product containing small controlled amounts of certain elements, such as boron or phosphorus, which impart desired semiconductor properties.

This invention has several advantages over prior art processes of melting and solidifying silicon. First, the danger of contamination is greatly reduced because the molten silicon is, in effect, handled by a silicon container. Second, the molten silicon is not solidified within the container so that adhesion or sticking between the solidified silicon and the container is eliminated.

sible to produce silicon rods of a relatively large diameter. The first specific example describes the production of a 2-inch rod without the need of a mold, and it is possible to produce even larger rods if desired.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

This case is a continuation-in-part of my copending application Ser. No. 568,890, filed March 1, 1956, now abandoned.

I claim:

1. An improved process for melting and solidifying silicon into a solid body of uniform composition comprising oonducting the following steps in a noncontaminating atmosphere: passing silicon into the high frequency electromagnetic field of an induction coil within an enclosure created by a nonconducting wall which is exteriorly contacted with a nonconducting fluid coolant to maintain its temperature below the melting point of silicon, said induction coil surrounding said wall, melting the silicon with the energyof said field, and continuously maintaining a pool of molten silicon within the enclosure, said pool being retained by said non-conducting wall and supported by solid silicon situated beneath the enclosure, touching the molten silicon with a seed of solid silicon and crystallizing molten silicon thereon, withdrawing said seed and the continuously crystallizing silicon through a cooling zone to the normal atmosphere and recovering a solid stick of silicon.

2. The process of claim 1 in which the seed of solid silicon is a single crystal.

3. An improved process for melting and solidifying silicon into a solid body of uniform composition comprising conducting the following steps in a noncontaminating atmosphere: passing a silicon rod upwardly into a high frequency electromagnetic field within an enclosure created by a nonconducting wall which is exteriorly contacted with a nonconducting fluid coolant to maintain its temperature below the melting point of silicon, said wall forming an enclosure which is slightly larger and of approximately the same shape as the silicon rod, melting the passing silicon rod with the energy of said field, and continuously maintaining a pool of molten silicon within the enclosure, said pool being retained by said nonconducting wall and supported by the unmelted portion of the passing rod, surface tension, and a higher gas pressure below the melt than above it, touching the surface of said pool with a seed of solid silicon and crystallizing molten silicon thereon, withdrawing said seed and the continously crystallizing silicon through a cooling zone to the normal atmosphere and recovering a solid stick of silicon.

4. An improved process for melting and solidifying silicon into a solid body of uniform composition comprising conducting the following steps in a noncontaminating atmosphere: passing silicon into a high frequency electromagnetic field within an enclosure created by a silica wall which is exteriorly contacted with water to maintain its temperature below 1100 C., melting the sili con with the energy of said field and continuously maintaining a pool of molten silicon within the enclosure, said pool being retained by said silica wall and supported by solid silicon, surface tension, and gas pressure, touching the molten silicon with a seed of solid silicon and crystallizing molten silicon thereon, withdrawing said seed and the continuously crystallizing silicon through a cooling zone to the normal atmosphere and recovering a solid stick of silicon.

5. The process of claim 4 in which the seed of solid silicon is a single crystal.

6. An improved process for melting and solidifying silicon into a solid body of uniform composition comprising conducting the following steps in a noncontaminating atmosphere! passing silicon downwardly into a high frequency electromagnetic field within an enclosure created by a nonconducting wall which is exteriorly contacted with a nonconducting fluid coolant to maintain its temperature below the melting point of silicon, continuously melting the silicon with the energy of said field and forming a pool of molten silicon within the enclosure of the nonconducting wall by maintaining beneath the lower end of said enclosure a solid silicon support and a gas pressure which is greater than the pressure over the molten pool, withdrawing said support downwardly through a cooling zone, thereby elongating said support with molten silicon which is continuously crystallizing thereon.

7. An improved process for melting and solidifying silicon into a solid body of uniform composition comprising conducting the following steps in a noncontaminating atmosphere: passing a silicon rod into a high frequency electromagnetic field within an enclosure created by a nonconducting wall which is exteriorly contacted with a nonconducting fluid coolant to maintain its temperature below the melting point of silicon, said wall forming an enclosure which is slightly larger and of approximately the same shape as the silicon rod, melting the passing silicon rod with the energy of said field, and continuously maintaining a pool of molten silicon with in the enclosure, said pool being retained by said nonconducting wall and supported by solid silicon, surface tension, and gas pressure, touching the molten silicon with a seed of solid silicon and withdrawing said seed from the molten silicon to a cooling zone at such a rate as to form a silicon rod of different cross-sectional size than the feed rod passing into the electromagnetic field and maintaining the volume of said pool substantially constant by passing the feed rod into said electromagnetic field at a rate sufficient to supply the pool with the same quantity of silicon as that being withdrawn.

Keck: The Rev. of Scien. Inst., vol. 25, #4, April 1954, pages 331-334.

Keck et al.: J. Applied Phys. 24, pages 1479-83, 1953. 

1. AN IMPROVED PROCESS FOR MELTING AND SOLIDIFYING SILICON INTO A SOLID BODY OF UNIFORM COMPOSITION COMPRISING CONDUITING THE FOLLOWING STEPS IN A NONCONTAMINATING ATMOSPHERE: PASSING SILICON INTO THE HIGH FREQUENCY ELECTROMAGNETIC FIELD OF AN INDUCTION COIL WITHIN AN ENCLOSURE CREATED BY A NONCONDUCTING WALL WHICH IS EXTERIORLY CONTACTED WITH A NONCONDUCTING FLUID COOLANT TO MAINTAIN ITS TEMPERATURE BELOW THE MELTING POINT OF SILICON, SAID INDUCTIONN COIL SURROUNDING SAID WALL, MELTING THE SILICON WITH THE ENERGY OF SAID FIELD, AND CON- 